Conventional nanosecond pulsed-laser melt annealing (“conventional melt laser annealing”) offers an ultra-low thermal budget, a high dopant activation and super-abrupt junctions that are ideal for advanced integrated circuit (IC) chip fabrication. In practice, however, it is difficult to implement this type of annealing on patterned wafers due to large temperature non-uniformities that can arise from spatial variations in the optical and thermal properties of the IC chip. These adverse effects are referred to in the art as “pattern density effects.”
Pattern density effects can be mitigated by using an absorber layer or a phase-switch layer. The absorber layer can significantly improve optical absorption uniformity, but the process window is still limited by the pattern density effects from inhomogeneous thermal properties. In particular, the short heat diffusion length (˜1 um) associated with the nanosecond pulse duration is not sufficient to average out spatial variations in the material properties of an IC chip during manufacturing.
For example, FIG. 1 shows a schematic cross-sectional view of a portion of a silicon substrate (wafer) 10 with a surface 11 that supports a patterned surface 12, with the patterned surface being subjected to conventional melt laser annealing. Patterned surface 12 includes a first poly gate G1 atop substrate surface 11 and a second poly gate G2 atop a field-oxide region 16 formed in substrate surface 11. An absorber layer 20 caps the two poly gates G1 and G2. A conventional laser annealing beam 30 scans across the wafer 10 in the direction of arrow 31 and heats the two poly gates G1 and G2.
FIG. 2 is a bar-graph plot of the maximum surface temperature TSM (K) of patterned wafer surface 12 at poly gates G1 and G2 as calculated by a computer simulation of the conventional melt laser annealing process. FIG. 2 shows how the capped poly gate G2 overheats due to poor thermal conductivity of the underlying field-oxide region 16, as compared to the poly gate G1 that resides directly on the silicon wafer. The bar-graph plot of FIG. 2 shows that such overheating due to pattern density effects can be more than 200° C. for the conventional melt laser annealing process. The addition of a phase-switch layer can mitigate the pattern density effects to some extent by self-regulating the light absorption at the overheated region. However, the use of such phase-switch layer introduces substantial process complexity and cost.